Had this issue show up again. I thought I'd never see it again. "Cannot write file -- disk may be full" After saving to a separate drive (even though my working drive was no place close to full), the file was not saved, and gone forever. Safe mode failed. How does something as advanced as Sonar X2 suddenly fail a task this simple? At least give me a button that I can click that says, "No, Sonar, YOU'RE full of it! Save the damn file." errr! Sorry I'm just venting. I don't need any help. The session work is gone. Fortunately the audio is still safe.

ANY disk write failure raises the same generic "disk may be full" message. I'd run a disk diagnostic on it just to be sure the drive isn't experiencing some kind of intermittent failure such as might be caused by a loose cable.

the first thing I would do though just to rule a few things out is run a virus scan using malwarebytes, not your anti virus, malwarebytes gets inside places typical AV cant, there is a free version.
I say this because there are viruses out there that will give the disk full error, that's part of the virus, to not let you be able to put another program on the disk.

second, I would defrag the HD, ive seen stranger things, but you didn't mention how much free space is on your HD or even if its a SSD, so im shooting blind here.

secong I would download a free version of a hard drive testing software, im not going to recommend one though.
don't know your HD, what it is?

I don't believe this is a Sonar problem. Sonar is telling Windows to save the file and Windows is telling Sonar, "sorry I can't, but I don't know why". Last time someone posted this I compiled a list of things to try to get past this error.