When you try to copy tape media in RSM, it complains that there is insufficient capacity and that you should use a unit with at least n.n GB of free capacity.
When a tape is manufactured, the actual capacity may vary several hundred megabytes. RSM attempts to compensate by keeping 5% free at the end of the tape. Look at the Free Space column under the Remote Storage Media icon. Because of the manufacturing variance, the amount of free space can be much less.
To workaround the problem, try to use media with a larger capacity. If you can't, you can alter the default MediaCopyTolerance, the percentage of capacity that the copy media is allowed to be smaller that the master. Use Regedt32 to navigate to:
On the Edit menu, Add value name MediaCopyTolerance, as a REG_SZ data type. The defaut is 2 percent and the recommended range is 2 - 5 percent.
You must shutdown and restart for this change to take effect.