Apparently conflicting backups causing crash

Hi

I have a ReadyNAS running ReadyOS 6.9.1 that is dedicated as a backup server to back up a couple of other units via Rsync. These backups are done every hour, 24/7. This has been working fine for some time.

I decided to add a backup to external disk of the shares on my backup server that contain the backups from my other units. However, the first time I tried this, the machine appeared to crash. No communication, etc. I power cycled it and everything seems to be ok, but the log entries basically stop right before the time the backup to disk would have been done.

I can't think of a way to prove it, but I suspect the problem is that the Rsync backups *to* my backup server are running at the same time as the backup to disk *on* my backup server, i.e clients are writing to shares that are being read at the same time, and there is a conflict. Unfortunately there appears to be no way to schedule backups at any time other than at 5 minutes past the hour. So the only way I know of to avoid this situation is to introduce a gap into my schedule for backing up my clients while the backup server is backing up to disk. This wouldn't be a huge problem, but I'd rather not do that if I can avoid it. 

So, I was wondering if there is a way to schedule backups on a more flexible schedule, or some other trick to avoid any potential conflict.

thanks