You are correct, volume backup of C: to save in D:\VeeamBackups.
I ended up using Windows Backup to backup my Win 7 Pro install, then I upgraded to Win 10 Pro. Upgrade went smoothly. Now in Win 10 I still have it installed and I tried to run it again to get a backup of Win 10, but its the same exact error.
Windows has no issue seeing the partition as it upgraded properly, so maybe an update to Veeam Endpoint you could just have Veeam use whatever engine or dll's Windows uses to manage locating where data on the disk resides?
I have a picture of the diskmgmt console but I can't figure out how to attach it here. Basically it looks like this
Disk0 (Drive D:) raid0 1192.32 GB NTFS Healthy (Active, Primary Partition)
Disk1 System Reserved 100 MB NTFS Healthy (System, Active) | Samsung PRO 840 SSD Drive C: 237.94 GB NTFS Healthy (Boot, Page File, Crash Dump, Primary Partition) | 450 MB Healthy (Recovery Partition)
Disk2 WD Passport (F:) 111.79 GB NTFS Healthy (Primary Partition) USB 2.0 drive
Drive G DVD-ROM
I am not sure where the system reserved or recovery partition came from on drive C: but I can assure you I did not create it. This is a custom build PC that I put together in 2010 from components purchaed at Newegg. Everything (CPU, board, memory, etc..) Over the years I just made the raid the D: drive and installed the 250 GB SSD, also upgraded the ram from 6GB to 12GB. Other than that it runs great. Windows must have created those other partitions, and if so it must be common and something Veeam needs to take in account for.
I just checked my laptop which is running Windows 8.1. It is a Sony Vaio and its C: drive has a 13.98 Recovery partition | System Reserved 100 MB NTFS | 224.41 GB NTFS (Bitlocker Encrypted) Healthy (Boot, Page File, Crash Dump, Primary Partition). I have not tried Veeam endpoint on this system yet.