Volumes On Dynamic Disk Not Accessible

I have a 2 TB eSATA drive that contains my backups. This drive has been working well for years and isn't accumulating errors. It was a basic disk until I deleted a partition that I wasn't using any more and expanded the size of the backup partition. That worked fine until yesterday when I was no longer able to access either of the partitions on the drive. Here's the output of the disk manager relating to this drive:

When I use Windows Explorer to access either of the volumes on the drive, I get a message "Z: is not accessible". When I click on Computer, I see both of the volumes on the disk, but there is no information regarding disk space used/available. If I right click on the drive and go Properties/Tools/Error Check, I get a message "The disk check cannot be performed because windows can not access the disk."

However, if I go into Disk Manage, right click on the partition and go to Properties/Tools/Error Check, I can scan the drive! It completes showing that it found the following errors:

Chkdsk was executed in scan mode on a volume snapshot.

Checking file system on Z:
Volume label is Backup_Files.

Stage 1: Examining basic file system structure ...

Stage 2: Examining file name linkage ...

Stage 3: Examining security descriptors ...

Found corrupt security descriptor entry at offset 0x80000 in $Secure <0,0x9>:$SDS ... queued for offline repair.[repeated 50 times]

[code].......

I then get a message that I need to reboot the computer to repair these errors, which I do. According to the event log, it repairs all of the errors, but I'm back at the same place that I was before and a scandisk of the volume shows the same errors.