Topic: problem with backup

[mactv]

I've been running sme-server v10 for 6 mths now; It was a new clean install on a new server; only ibay data migrated from previous v9 server.
I've been running a backup to workstation where the backup target was an internal drive on another v10 system. The backup was configured to have 4 sets of 1 full + 29 increments each. This backup was working OK.
I tried running a backup to workstation where the backup target was a mounted external drive attached by USB  and with the same 4 sets of 1 full + 29 increments. This backup appeared to only create full backups each day and quickely filled up the external drive to capacity. The system had problems; I noticed on Disk Usage that \run was 100%; I rebooted the system  and \run looked OK at around 1%.
Since then whenever I try to run a backup to either the mounted USB attached drive or to the other V10 system I get an error saying \run is busy and backup is already mounted.
Can anyone advise on a) how to eliminate this message and b) whether a backup to a mounted drive is possible?

Thanks