Unable to back up disk X: due to invalid partition table

The error is caused by a volume type with no partition table and is not supported by Veeam Agent for Microsoft Windows.

In Windows Disk Management, the volume looks like a common MBR single partition. The Virtual Disk service, which Veeam Endpoint Backup uses to detect disks, generates the following messages that are then forwarded to the Veeam log. (log location: C:\ProgramData\Veeam\Backup\Endpoint\Backup_Job_*Machine name*\ Job.Backup_Job_Machine_name.Backup.log).
 

Info     Layout: MBR, disk signature: 1
Info     Partition 0, type 04 (FAT16), offset 0

The MBR partition table occupies sector 1 of the disk, so Partition 0 can start no earlier than sector 2. One sector equals 512 bytes for regular disks and 4096 bytes for 4K native hard disk drives. Meaning the offset of Partition 0 must be greater than 512 or 4096.

1. Copy all data from the affected volume to another drive, because the workaround implies complete deletion of the volume in question. 
2. Open disk management > right click on the volume in question> delete volume. 
3. Then create a new volume and copy all the files back to it. 
4. After that you should be able to run the backup job successfully.