Issues with root: Cross-device link

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Before I begin, I just want to say that I've done my due diligence and searched for an answer for this... both on xda and the interwebs.

I have successfully obtained root on my phone... did it about 4 days ago. At the same time, I was able to successfully move all of my unnecessary HTC provided widgets such as Stock.apk to a folder I created within the /system/app location. But now, I'm having a problem with root access.

I have mounted the phone with the command:
# mount -o remount,rw -t yaffs2 /dev/block/mtdblock3 /system
but yet when I attempt the following command:
# mv HtcClockWidget.apk /system/app
I get the following error:
failed on 'HtcClockWidget.apk' - Cross-device link

I've rerooted the phone three times now, which is sufficient enough for me to go to you guys and ask. I think I may be doing something wrong. Any ideas? I'm totally lost.

EDIT:
To clarify, I'm attempting to replace the original HtcClockWidget.apk from Sprint to the flip clock from a GSM Hero. I have moved the HtcClockWidget.apk original to /system/app/ap for backup purposes. Upon moving the file and attempting to move the GSM file to the /system/app folder, I get the Cross-device link error. But, if I move the HtcClockWidget.apk original file back from the /ap backup folder to its original location, it has no problem doing so. Is there some kind of protection that the /system/app folder has that can distinguish between two different HtcClockWidget.apk's?

are you using MoDaCo's rom? if so just use cp HtcClockWidget.apk

and is your pwd your sd card?

No, I am using the stock ROM. I just so happened to stumble across this thread:

http://forum.xda-developers.com/showthread.php?t=554826

I guess I can only replace the /system/app files while in recovery with the /system directory mounted! Problem solved!

do you have the gsm htc clock (with animation) apk????

is that why I dont see any animations on my flip clock? Always wondered because my Dream Hero always had (albeit slow) animations. Btw, I seem to be able to mount the system as read/write even with the phone booted normally.