Timeshift problem; disk unmounts itself.

I've been running Timeshift since way back in 2018 with the data stored on an external USB 500Gb disk I rescued from my old Virgin Media box. (When it was upgraded they didn't want the old box back, recycle it they said so I kept the disk and chucked the rest.) The disk has two partitions, one 315Gb EXT4 used for timeshift and a 185Gb NTFS one used for general storage. I configured Timeshift to run weekly snapshots and hold on to three over and above the original one full manually created one.
This has been working fine for the last couple of years but now when it runs, either scheduled or manually instigated, everything appears to be working then the disk (both partitions) suddenly disappears from the desktop with a loud "beep" and Timestamp sits twiddling its thumbs for a while. The partitions eventually re-appear but no data is saved. See the rather long dmesg output below:

Code:[17863.399213] [UFW BLOCK] IN=enp1s0 OUT= MAC=01:00:5e:00:00:fb:a8:23:fe:10:49:c8:08:00 SRC=192.168.0.35 DST=224.0.0.251 LEN=32 TOS=0x00 PREC=0xC0 TTL=1 ID=0 DF PROTO=2
[17985.177716] [UFW BLOCK] IN=enp1s0 OUT= MAC=01:00:5e:00:00:01:48:d3:43:05:41:68:08:00 SRC=192.168.0.1 DST=224.0.0.1 LEN=32 TOS=0x00 PREC=0xC0 TTL=1 ID=52457 PROTO=2
[17989.912524] [UFW BLOCK] IN=enp1s0 OUT= MAC=01:00:5e:00:00:fb:a8:23:fe:10:49:c8:08:00 SRC=192.168.0.35 DST=224.0.0.251 LEN=32 TOS=0x00 PREC=0xC0 TTL=1 ID=0 DF PROTO=2
[18076.880870] usb 2-4: reset high-speed USB device number 6 using ehci-pci
[18110.176739] [UFW BLOCK] IN=enp1s0 OUT= MAC=01:00:5e:00:00:01:48:d3:43:05:41:68:08:00 SRC=192.168.0.1 DST=224.0.0.1 LEN=32 TOS=0x00 PREC=0xC0 TTL=1 ID=52465 PROTO=2
[18114.889946] [UFW BLOCK] IN=enp1s0 OUT= MAC=01:00:5e:00:00:fb:a8:23:fe:10:49:c8:08:00 SRC=192.168.0.35 DST=224.0.0.251 LEN=32 TOS=0x00 PREC=0xC0 TTL=1 ID=0 DF PROTO=2
[18121.669467] usb 2-4: USB disconnect, device number 6
[18121.669929] sd 7:0:0:0: Device offlined - not ready after error recovery
[18121.684316] sd 7:0:0:0: [sdg] tag#0 FAILED Result: hostbyte=DID_NO_CONNECT driverbyte=DRIVER_OK
[18121.684323] sd 7:0:0:0: [sdg] tag#0 CDB: Write(10) 2a 00 15 d1 ab 40 00 00 f0 00
[18121.684326] print_req_error: I/O error, dev sdg, sector 366062400
[18121.684402] EXT4-fs warning (device sdg2): ext4_end_bio:323: I/O error 10 writing to inode 658955 (offset 41943040 size 4194304 starting block 45758464)
[18121.684405] buffer_io_error: 4742 callbacks suppressed
[18121.684407] Buffer I/O error on device sdg2, logical block 582144
[18121.684415] Buffer I/O error on device sdg2, logical block 582145
[18121.684417] Buffer I/O error on device sdg2, logical block 582146
[18121.684419] Buffer I/O error on device sdg2, logical block 582147
[18121.684421] Buffer I/O error on device sdg2, logical block 582148
[18121.684423] Buffer I/O error on device sdg2, logical block 582149
[18121.684424] Buffer I/O error on device sdg2, logical block 582150
[18121.684426] Buffer I/O error on device sdg2, logical block 582151
[18121.684428] Buffer I/O error on device sdg2, logical block 582152
[18121.684430] Buffer I/O error on device sdg2, logical block 582153
[18121.684751] sd 7:0:0:0: [sdg] tag#0 FAILED Result: hostbyte=DID_NO_CONNECT driverbyte=DRIVER_OK
[18121.684753] sd 7:0:0:0: [sdg] tag#0 CDB: Write(10) 2a 00 15 d1 ac 30 00 00 f0 00
[18121.684754] print_req_error: I/O error, dev sdg, sector 366062640
[18121.684757] EXT4-fs warning (device sdg2): ext4_end_bio:323: I/O error 10 writing to inode 658955 (offset 41943040 size 4194304 starting block 45757952)
[18121.685178] JBD2: Detected IO errors while flushing file data on sdg2-8
[18121.685186] Aborting journal on device sdg2-8.
[18121.685211] JBD2: Error -5 detected when updating journal superblock for sdg2-8.
[18121.685384] EXT4-fs error (device sdg2) in ext4_reserve_inode_write:5861: Journal has aborted
[18121.685417] EXT4-fs warning (device sdg2): ext4_end_bio:323: I/O error 10 writing to inode 658956 (offset 41943040 size 4194304 starting block 45760000)
[18121.685625] EXT4-fs error (device sdg2): mpage_map_and_submit_extent:2586: comm kworker/u32:6: Failed to mark inode 658956 dirty
[18121.685629] EXT4-fs (sdg2): previous I/O error to superblock detected
[18121.685646] EXT4-fs error (device sdg2) in ext4_writepages:2915: Journal has aborted
[18121.685649] EXT4-fs (sdg2): previous I/O error to superblock detected
[18121.685719] EXT4-fs warning (device sdg2): ext4_end_bio:323: I/O error 10 writing to inode 658956 (offset 41943040 size 4194304 starting block 45760512)
[18121.686126] JBD2: Detected IO errors while flushing file data on sdg2-8
[18121.686211] EXT4-fs (sdg2): previous I/O error to superblock detected
[18121.686232] EXT4-fs error (device sdg2): ext4_journal_check_start:61: Detected aborted journal
[18121.686234] EXT4-fs (sdg2): Remounting filesystem read-only
[18121.686236] EXT4-fs (sdg2): previous I/O error to superblock detected
[18121.686252] EXT4-fs (sdg2): ext4_writepages: jbd2_start: 9223372036854775807 pages, ino 658957; err -30
[18122.739418] [UFW BLOCK] IN=enp1s0 OUT= MAC= SRC=fe80:0000:0000:0000:aa0a:3a35:0671:cd01 DST=ff02:0000:0000:0000:0000:0000:0000:0001 LEN=64 TC=0 HOPLIMIT=1 FLOWLBL=69058 PROTO=UDP SPT=8612 DPT=8612 LEN=24
[18122.739439] [UFW BLOCK] IN=enp1s0 OUT= MAC= SRC=fe80:0000:0000:0000:aa0a:3a35:0671:cd01 DST=ff02:0000:0000:0000:0000:0000:0000:0001 LEN=64 TC=0 HOPLIMIT=1 FLOWLBL=598407 PROTO=UDP SPT=8612 DPT=8610 LEN=24
[18122.749616] [UFW BLOCK] IN=enp1s0 OUT= MAC= SRC=fe80:0000:0000:0000:aa0a:3a35:0671:cd01 DST=ff02:0000:0000:0000:0000:0000:0000:0001 LEN=64 TC=0 HOPLIMIT=1 FLOWLBL=69058 PROTO=UDP SPT=8612 DPT=8612 LEN=24
[18122.749641] [UFW BLOCK] IN=enp1s0 OUT= MAC= SRC=fe80:0000:0000:0000:aa0a:3a35:0671:cd01 DST=ff02:0000:0000:0000:0000:0000:0000:0001 LEN=64 TC=0 HOPLIMIT=1 FLOWLBL=598407 PROTO=UDP SPT=8612 DPT=8610 LEN=24
[18129.571254] blk_partition_remap: fail for partition 2
[18129.571260] EXT4-fs error (device sdg2): ext4_wait_block_bitmap:532: comm rsync: Cannot read block bitmap - block_group = 12, block_bitmap = 1037
[18129.571265] EXT4-fs error (device sdg2): ext4_discard_preallocations:4129: comm rsync: Error -5 reading block bitmap for 12
[18136.660145] usb 2-4: new high-speed USB device number 7 using ehci-pci
[18136.817711] usb 2-4: New USB device found, idVendor=152d, idProduct=2338
[18136.817714] usb 2-4: New USB device strings: Mfr=1, Product=2, SerialNumber=5
[18136.817716] usb 2-4: Product: USB to ATA/ATAPI bridge
[18136.817717] usb 2-4: Manufacturer: JMicron
[18136.817718] usb 2-4: SerialNumber: 000001D91CA0
[18136.818589] usb-storage 2-4:1.0: USB Mass Storage device detected
[18136.818763] scsi host7: usb-storage 2-4:1.0
[18137.037948] [UFW BLOCK] IN=enp1s0 OUT= MAC= SRC=fe80:0000:0000:0000:aa0a:3a35:0671:cd01 DST=ff02:0000:0000:0000:0000:0000:0000:0001 LEN=64 TC=0 HOPLIMIT=1 FLOWLBL=69058 PROTO=UDP SPT=8612 DPT=8612 LEN=24
[18137.037964] [UFW BLOCK] IN=enp1s0 OUT= MAC= SRC=fe80:0000:0000:0000:aa0a:3a35:0671:cd01 DST=ff02:0000:0000:0000:0000:0000:0000:0001 LEN=64 TC=0 HOPLIMIT=1 FLOWLBL=598407 PROTO=UDP SPT=8612 DPT=8610 LEN=24
[18137.048242] [UFW BLOCK] IN=enp1s0 OUT= MAC= SRC=fe80:0000:0000:0000:aa0a:3a35:0671:cd01 DST=ff02:0000:0000:0000:0000:0000:0000:0001 LEN=64 TC=0 HOPLIMIT=1 FLOWLBL=69058 PROTO=UDP SPT=8612 DPT=8612 LEN=24
[18137.048259] [UFW BLOCK] IN=enp1s0 OUT= MAC= SRC=fe80:0000:0000:0000:aa0a:3a35:0671:cd01 DST=ff02:0000:0000:0000:0000:0000:0000:0001 LEN=64 TC=0 HOPLIMIT=1 FLOWLBL=598407 PROTO=UDP SPT=8612 DPT=8610 LEN=24
[18137.910922] scsi 7:0:0:0: Direct-Access WDC WD50 00AAKS-00TMA0 PQ: 0 ANSI: 5
[18137.911499] sd 7:0:0:0: Attached scsi generic sg7 type 0
[18137.912191] sd 7:0:0:0: [sdg] 976773168 512-byte logical blocks: (500 GB/466 GiB)
[18137.913225] sd 7:0:0:0: [sdg] Write Protect is off
[18137.913228] sd 7:0:0:0: [sdg] Mode Sense: 28 00 00 00
[18137.914221] sd 7:0:0:0: [sdg] No Caching mode page found
[18137.914225] sd 7:0:0:0: [sdg] Assuming drive cache: write through
[18137.935668] sdg: sdg1 sdg2
[18137.939109] sd 7:0:0:0: [sdg] Attached SCSI disk
[18148.689104] EXT4-fs (sdg2): recovery complete
[18148.689777] EXT4-fs (sdg2): mounted filesystem with ordered data mode. Opts: (null)I can read and write data to the NTFS partition but the EXT4 one is owned by root so read only to me as a user, is this normal for Timeshift or is it because the partition was remounted read only as per the dmesg output above?
Does it have to be available to the user as Read/Write for Timeshift to work?

I've tried reformatting the EXT partition but the disk still gets dropped during a snapshot.

The disk itself appears to be fine, see Smartctrl report below:
Quote:

smartctl 6.5 2016-01-24 r4214 [x86_64-linux-4.15.0-106-generic] (local build) Copyright (C) 2002-16, Bruce Allen, Christian Franke, www.smartmontools.org === START OF INFORMATION SECTION === Model Family: Western Digital Caviar Blue (SATA) Device Model: WDC WD5000AAKS-00TMA0 Serial Number: WD-WCAPW4347673 LU WWN Device Id: 5 0014ee 1aaeb7a86 Firmware Version: 12.01C01 User Capacity: 500,107,862,016 bytes [500 GB] Sector Size: 512 bytes logical/physical Device is: In smartctl database [for details use: -P show] ATA Version is: ATA/ATAPI-7 (minor revision not indicated) Local Time is: Tue Jun 30 11:41:24 2020 BST SMART support is: Available - device has SMART capability. SMART support is: Enabled === START OF READ SMART DATA SECTION === SMART overall-health self-assessment test result: PASSED General SMART Values: Offline data collection status: (0x82)Offline data collection activity was completed without error. Auto Offline Data Collection: Enabled. Self-test execution status: ( 0)The previous self-test routine completed without error or no self-test has ever been run. Total time to complete Offline data collection: (12000) seconds. Offline data collection capabilities: (0x7b) SMART execute Offline immediate. Auto Offline data collection on/off support. Suspend Offline collection upon new command. Offline surface scan supported. Self-test supported. Conveyance Self-test supported. Selective Self-test supported. SMART capabilities: (0x0003)Saves SMART data before entering power-saving mode. Supports SMART auto save timer. Error logging capability: (0x01)Error logging supported. General Purpose Logging supported. Short self-test routine recommended polling time: ( 2) minutes. Extended self-test routine recommended polling time: ( 150) minutes. Conveyance self-test routine recommended polling time: ( 6) minutes. SCT capabilities: (0x303f)SCT Status supported. SCT Error Recovery Control supported. SCT Feature Control supported. SCT Data Table supported. SMART Attributes Data Structure revision number: 16 Vendor Specific SMART Attributes with Thresholds: ID# ATTRIBUTE_NAME FLAG VALUE WORST THRESH TYPE UPDATED WHEN_FAILED RAW_VALUE 1 Raw_Read_Error_Rate 0x000f 200 200 051 Pre-fail Always - 0 3 Spin_Up_Time 0x0003 173 168 021 Pre-fail Always - 6341 4 Start_Stop_Count 0x0032 096 096 000 Old_age Always - 4457 5 Reallocated_Sector_Ct 0x0033 200 200 140 Pre-fail Always - 0 7 Seek_Error_Rate 0x000e 200 200 051 Old_age Always - 0 9 Power_On_Hours 0x0032 065 065 000 Old_age Always - 26082 10 Spin_Retry_Count 0x0012 100 100 051 Old_age Always - 0 11 Calibration_Retry_Count 0x0012 100 100 051 Old_age Always - 0 12 Power_Cycle_Count 0x0032 096 096 000 Old_age Always - 4274 192 Power-Off_Retract_Count 0x0032 199 199 000 Old_age Always - 922 193 Load_Cycle_Count 0x0032 199 199 000 Old_age Always - 4481 194 Temperature_Celsius 0x0022 130 100 000 Old_age Always - 20 196 Reallocated_Event_Count 0x0032 200 200 000 Old_age Always - 0 197 Current_Pending_Sector 0x0012 200 200 000 Old_age Always - 0 198 Offline_Uncorrectable 0x0010 200 200 000 Old_age Offline - 0 199 UDMA_CRC_Error_Count 0x003e 200 200 000 Old_age Always - 24 200 Multi_Zone_Error_Rate 0x0008 200 200 051 Old_age Offline - 0 SMART Error Log Version: 1 No Errors Logged SMART Self-test log structure revision number 1 Num Test_Description Status Remaining LifeTime(hours) LBA_of_first_error # 1 Extended offline Completed without error 00% 26072 - # 2 Extended offline Completed without error 00% 25873 - # 3 Extended offline Interrupted (host reset) 60% 25834 - # 4 Extended offline Interrupted (host reset) 80% 25834 - # 5 Conveyance offline Completed without error 00% 25826 - # 6 Short offline Completed without error 00% 25824 - SMART Selective self-test log data structure revision number 1 SPAN MIN_LBA MAX_LBA CURRENT_TEST_STATUS 1 0 0 Not_testing 2 0 0 Not_testing 3 0 0 Not_testing 4 0 0 Not_testing 5 0 0 Not_testing Selective self-test flags (0x0): After scanning selected spans, do NOT read-scan remainder of disk. If Selective self-test is pending on power-up, resume after 0 minute delay.

The two incomplete Extended tests was me giving up at one o'clock in the morning a couple of times and shutting down.

I'm running Mint 18.3 on my Z400 Workstation. Kernel 4.15.0-107-generic at present though the problem happened with several previous versions as well.

Any advice on where I should go now would be appreciated,

Play Bonny!

:hattip: