Thank you all for the replies!
Unfortunately a little while ago Unraid disconnected the two disks attached to the expansion card.
I'm afraid I need to replace this board, it doesn't seem to be working properly. Here are the recent logs of the two disks:
DISK 1
Mar 12 19:37:07 tomato kernel: ata6.00: failed command: READ FPDMA QUEUED
Mar 12 19:37:07 tomato kernel: ata6.00: cmd 60/18:b0:28:9b:e7/00:00:74:00:00/40 tag 22 ncq dma 12288 in
Mar 12 19:37:07 tomato kernel: ata6.00: status: { DRDY }
Mar 12 19:37:07 tomato kernel: ata6: hard resetting link
Mar 12 19:37:12 tomato kernel: ata6: link is slow to respond, please be patient (ready=0)
Mar 12 19:37:17 tomato kernel: ata6: softreset failed (device not ready)
Mar 12 19:37:17 tomato kernel: ata6: hard resetting link
Mar 12 19:37:27 tomato kernel: ata6: softreset failed (1st FIS failed)
Mar 12 19:37:27 tomato kernel: ata6: hard resetting link
Mar 12 19:38:02 tomato kernel: ata6: softreset failed (1st FIS failed)
Mar 12 19:38:02 tomato kernel: ata6: limiting SATA link speed to 3.0 Gbps
Mar 12 19:38:02 tomato kernel: ata6: hard resetting link
Mar 12 19:38:07 tomato kernel: ata6: softreset failed (1st FIS failed)
Mar 12 19:38:07 tomato kernel: ata6: reset failed, giving up
Mar 12 19:38:07 tomato kernel: ata6.00: disable device
Mar 12 19:38:07 tomato kernel: ata6: EH complete
Mar 12 19:38:07 tomato kernel: sd 6:0:0:0: [sdd] tag#2 UNKNOWN(0x2003) Result: hostbyte=0x04 driverbyte=DRIVER_OK cmd_age=90s
Mar 12 19:38:07 tomato kernel: sd 6:0:0:0: [sdd] tag#2 CDB: opcode=0x28 28 00 74 e7 9b 28 00 00 18 00
Mar 12 19:38:07 tomato kernel: I/O error, dev sdd, sector 1961335592 op 0x0:(READ) flags 0x0 phys_seg 3 prio class 2
Mar 12 19:38:07 tomato kernel: sd 6:0:0:0: [sdd] tag#3 UNKNOWN(0x2003) Result: hostbyte=0x04 driverbyte=DRIVER_OK cmd_age=90s
Mar 12 19:38:07 tomato kernel: sd 6:0:0:0: [sdd] tag#3 CDB: opcode=0x28 28 00 74 e7 9a b8 00 00 70 00
Mar 12 19:38:07 tomato kernel: I/O error, dev sdd, sector 1961335480 op 0x0:(READ) flags 0x0 phys_seg 14 prio class 2
Mar 12 19:38:07 tomato kernel: sd 6:0:0:0: [sdd] tag#4 UNKNOWN(0x2003) Result: hostbyte=0x04 driverbyte=DRIVER_OK cmd_age=90s
Mar 12 19:38:07 tomato kernel: sd 6:0:0:0: [sdd] tag#4 CDB: opcode=0x28 28 00 74 e7 9a 98 00 00 10 00
Mar 12 19:38:07 tomato kernel: I/O error, dev sdd, sector 1961335448 op 0x0:(READ) flags 0x0 phys_seg 2 prio class 2
Mar 12 19:38:07 tomato kernel: sd 6:0:0:0: [sdd] tag#5 UNKNOWN(0x2003) Result: hostbyte=0x04 driverbyte=DRIVER_OK cmd_age=90s
Mar 12 19:38:07 tomato kernel: sd 6:0:0:0: [sdd] tag#5 CDB: opcode=0x28 28 00 74 e7 9a 80 00 00 08 00
Mar 12 19:38:07 tomato kernel: I/O error, dev sdd, sector 1961335424 op 0x0:(READ) flags 0x0 phys_seg 1 prio class 2
Mar 12 19:38:07 tomato kernel: sd 6:0:0:0: [sdd] tag#6 UNKNOWN(0x2003) Result: hostbyte=0x04 driverbyte=DRIVER_OK cmd_age=90s
Mar 12 19:38:07 tomato kernel: sd 6:0:0:0: [sdd] tag#6 CDB: opcode=0x28 28 00 74 e7 9b 40 00 00 40 00
Mar 12 19:38:07 tomato kernel: I/O error, dev sdd, sector 1961335616 op 0x0:(READ) flags 0x0 phys_seg 8 prio class 2
Mar 12 19:38:07 tomato kernel: sdd: detected capacity change from 3907029168 to 0
Mar 12 19:38:07 tomato kernel: sd 6:0:0:0: [sdd] tag#0 access beyond end of device
DISK2
Mar 12 19:39:48 tomato kernel: ata7: softreset failed (1st FIS failed)
Mar 12 19:39:58 tomato kernel: ata7: softreset failed (1st FIS failed)
Mar 12 19:40:33 tomato kernel: ata7: softreset failed (1st FIS failed)
Mar 12 19:40:33 tomato kernel: ata7: limiting SATA link speed to 3.0 Gbps
Mar 12 19:40:38 tomato kernel: ata7: softreset failed (1st FIS failed)
Mar 12 19:40:38 tomato kernel: ata7: reset failed, giving up
Mar 12 19:40:38 tomato kernel: ata7.00: disable device
Mar 12 19:40:38 tomato kernel: sde: detected capacity change from 3907029168 to 0