'media manager received no data for backup image' backing up USB drive

We have a Windows 2008 R2 server setup as a media server that backs up smaller clients. The server has a 2tb USB drive attached that contains all our installation software and appears as a local F: drive. Although I can browse the contents of F: in the backup policy 'Backup Selections' all attempts to backup data from the drive fail. Other ordinary local drives backup fine.

18/12/2016 00:00:58 - Info nbjm(pid=5216) starting backup job (jobid=5029922) for client deploy01.corpadds.net, policy MBFS_FS_DEPLOY01_DEV, schedule Weekly_Full
18/12/2016 00:01:00 - estimated 2258934 Kbytes needed
18/12/2016 00:01:00 - Info nbjm(pid=5216) started backup (backupid=deploy01.corpadds.net_1482019260) job for client deploy01.corpadds.net, policy MBFS_FS_DEPLOY01_DEV, schedule Weekly_Full on storage unit MKDD002_MBFS_DEPLOY01
18/12/2016 00:01:02 - started process bpbrm (4528)
18/12/2016 00:01:03 - Info bpbrm(pid=4528) deploy01.corpadds.net is the host to backup data from
18/12/2016 00:01:03 - Info bpbrm(pid=4528) reading file list from client
18/12/2016 00:01:03 - Info bpbrm(pid=4528) starting bpbkar32 on client
18/12/2016 00:01:03 - connecting
18/12/2016 00:01:03 - connected; connect time: 0:00:00
18/12/2016 00:01:06 - Info bpbkar32(pid=3640) Backup started
18/12/2016 00:01:06 - Info bptm(pid=4028) start
18/12/2016 00:01:06 - Info bptm(pid=4028) using 262144 data buffer size
18/12/2016 00:01:06 - Info bptm(pid=4028) setting receive network buffer to 1049600 bytes
18/12/2016 00:01:06 - Info bptm(pid=4028) using 30 data buffers
18/12/2016 00:01:10 - Info bptm(pid=4028) start backup
18/12/2016 00:01:10 - begin writing
18/12/2016 00:01:27 - Info bpbkar32(pid=3640) change journal NOT enabled for <F:\Support\12. Symantec>
18/12/2016 00:01:27 - Info bptm(pid=4028) waited for full buffer 1 times, delayed 1068 times
18/12/2016 00:01:27 - Info bpbkar32(pid=3640) bpbkar waited 0 times for empty buffer, delayed 0 times.
18/12/2016 00:01:27 - Info bptm(pid=4028) EXITING with status 90 <----------
18/12/2016 00:01:31 - Error bpbrm(pid=4528) cannot send mail to [snip]
18/12/2016 00:01:31 - Info bpbkar32(pid=3640) done. status: 90: media manager received no data for backup image
18/12/2016 00:01:31 - end writing; write time: 0:00:21
media manager received no data for backup image (90)

How can I make this work? We need a weekly backup of this data. The target server is running Netbackup 7.5.0.6, the master is at 7.6.1.2. But this still didn't work back when everything was at 7.5.0.6. 

Thanks