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I tried the update.zip, won't work, said verification failed or something like that.
What do I do?
Thanks.Just download an existing update that works and place your buildprop in the right place/folder and that should do it.
None of your suggestions worked...
I DO however, have the original build.prop on my computer, I can't adb push it to the phone, but could I put in an update.zip and then boot my phone in recovery mode?
Why can't you adb push it to your phone?
Might be easier to troubleshoot adb than to build an update.zip.
Thanks. Apparently I had used an old/non-working method to sign my ZIP.Gotcha.
Building an update.zip doesn't appear to be too hard, but it does require you to sign it. I tried doing this a while back but actually never got it to work right and didn't put much time into it. However, what was failing for me was not the signature verification part, something was flaking out in the somewhat-complex update.zip I was putting together. In retrospect, I should have tried a simple one first, before attempting what I was screwing around with.
Check this link:
How to Create Android Update Zip Package | All About Web & Mobile Application Development
update.zip didn't work, same issue with the verification
I hate to make any of you guys do work, but could any of you try verifying it yourselves? I'm new to rooting and all, so I may have done something wrong.
The update.zip I want to use is a few posts above.

(this uploaded file has your Milestone X build.prop in it). It doesn't look like your update.zip is signed. Also it looks like you are setting permissions for the "app" folder in /system instead of the build.prop.
Try this, it worked on my X with my build.prop of course(this uploaded file has your Milestone X build.prop in it).
Let us know how it goes.
update_build_prop_MX.zip
Are you trying to make an update.zip to fix the screen density issue?
Can you open any Apps and get them to function?
Instead of the verification error I get:
"E:failed to verily whole-file signature"
"E:signature verification failed"
Installation aborted.
EDIT:
On the fourth try I get "E:Can't open /cache/recovery/command"
Are you using CWR to apply the file?
The milestone might require different method/signing than the X does. Do you have a mod zip for the milestone (a known working one) that we can examine?