Backup to external drives with Acronis Backup 12.5

I've setup backup plans in Acronis Backup 12.5, that backups data from our server to external USB hard drives, the external drives have 4 partitions. I have 4 backup plans, each backing up to a partition on the external hard drive. Whilst the backup to the first partition works fine, it fails when running the backup plans for the next 3 partitions. If I change the path of the plan to the backup location of the first partition, it always works, whereas if I try to backup to any of the other 3 partition, I get a error message the Acronis Backup cannot locate the file:

2019-10-04T14:23:22:706+01:00 13012 I0135003A: User is running command. Command=Backup plan 'Week2'; User=**********************\administrator; clientProfileID=B57FFE58-D8E4-2F2A-229D-162EF0EDDC22; clientSessionID=A9645224-78A4-499E-A781-99C774FE8E12; tenantID=00000000-0000-0000-0000-000000000000
2019-10-04T14:23:23:077+01:00 13012 E0135003D: Error 0x135003d: Command has failed. Command=Backup plan 'Week2'; tenantID=00000000-0000-0000-0000-000000000000
| trace level: error
| channel: tol-activity#7FE69C84-B430-4E08-AAF4-3D0711BE9B36
| line: 0x4a8728dc8a1c9584
| file: e:\528\enterprise\common\tol\gating_activity.cpp:222
| function: Tol::`anonymous-namespace'::BusinessActivityTracker::OnCompleted
| $module: mms_vsa64_14330
|
| error 0x1350016: TOL: Failed to execute the command. Backup plan 'Week2'
| line: 0x8d165e86fb819597
| file: e:\528\enterprise\common\tol\command\command.cpp:457
| function: Tol::`anonymous-namespace'::MakeFailResult
| CommandID: D332948D-A7A9-4E07-B76C-253DCF6E17FB
| $module: mms_vsa64_14330
|
| error 0x1350016: TOL: Failed to execute the command. Backup plan 'Week2'
| line: 0x8d165e86fb819597
| file: e:\528\enterprise\common\tol\command\command.cpp:457
| function: Tol::`anonymous-namespace'::MakeFailResult
| CommandID: D332948D-A7A9-4E07-B76C-253DCF6E17FB
| $module: agent_protection_addon_vsa64_14330
|
| error 0x1330029: Failed to execute the command.
| line: 0xe6792a5ee190de2a
| file: e:\528\enterprise\managers\gtob\protection\agent_engine\protect_command.cpp:216
| function: `anonymous-namespace'::ProtectCommand::SafeExecute
| $module: agent_protection_addon_vsa64_14330
|
| error 0x1350016: TOL: Failed to execute the command. Checking sources and vault in backup plan
| line: 0x8d165e86fb819597
| file: e:\528\enterprise\common\tol\command\command.cpp:457
| function: Tol::`anonymous-namespace'::MakeFailResult
| CommandID: 6F33AD8F-B487-4B18-87A3-C5DBD7B77B95
| $module: mms_vsa64_14330
|
| error 0x1350016: TOL: Failed to execute the command. Checking sources and vault in backup plan
| line: 0x8d165e86fb819597
| file: e:\528\enterprise\common\tol\command\command.cpp:457
| function: Tol::`anonymous-namespace'::MakeFailResult
| CommandID: 6F33AD8F-B487-4B18-87A3-C5DBD7B77B95
| $module: gtob_ensure_backup_invariants_command_addon_vsa64_14330
|
| error 0x1330021: EnsureBackupInvariantsCommand: Failed to prepare the location for the backup archive.
| line: 0x5d2a9295cc9442e7
| file: e:\528\enterprise\managers\gtob\backupers\ensure_backup_invariants_command\impl\command.cpp:145
| function: `anonymous-namespace'::EnsureBackupInvariantsCommand::SafeExecute
| $module: gtob_ensure_backup_invariants_command_addon_vsa64_14330
|
| error 0x9a000d: The vault is unavailable.
| line: 0x9a79f20828bca60
| file: e:\528\enterprise\mms\managers\location\impl\manager.cpp:752
| function: LocationManagement::LocationManagerImpl::AddLocation
| IsReturnCode: 0x1
| $module: disk_bundle_vsa64_14330
|
| error 0x9a000d: The vault is unavailable.
| line: 0x90bbca5eaf5c5cb3
| file: e:\528\enterprise\mms\managers\location\impl\availability_checker.cpp:183
| function: `anonymous-namespace'::DefaultLocationChecker::CheckAvailability
| $module: disk_bundle_vsa64_14330
|
| error 0x40007: Error occurred while opening the file.
| line: 0xf35f747b3b21fabc
| file: e:\528\core\file\windows\winnt_dir.cpp:947
| function: winnt_dir::OpenWin32Dir
| $module: disk_bundle_vsa64_14330