Volume can't be converted

hello

i want to convert an apple raid 0 (three internal disks on my mac pro) to a SoftRAID volume and get the message

"the volume "xxx" can't be converted to a softraid volume. softraid can't convert this volume, because the hard disk, where the volume lies, is used by other volumes."

i have three hd's in raid 0, consisting of two volumes. one volume uses the tree inner parts (the slower) and one volume uses the three outer parts (faster) of the hard disks.

also one of the volumes is synchronized with a synology cloud.

any suggestions?

best, jorma.