Are you Migrating your Existing SCCM /Configuration Manager Infrastructure to Configuration manager 2012 ,if so, You will have to look at your Packages whether they UNC path or Local Drive Path since Migration of Packages using Local Drive(Non UNC) can not done.
Before you Migrate the Packages from CM07,you will have to ensure all of them are using UNC Path.
How to Know which are Local Drive ?
Here is Simple SQL query /SCCM report to know how many packages not using UNC Path.
select PkgID,Name,Version,Language,Source from SMSPackages where Source not like '\\%' and Source not like ''
Note:Please change the quotes,they are converted to fancy by Blog.
To change the Drive letter to UNC path,you can either use Coretech Tool or Scripting methods to update it.
The Consequences changing the Source Path:
You need to test and plan these package sources updates. Updating a package source will start a package update on all the distribution points. For even a medium size site this can take several hours.